1. Field of the Invention
The present invention relates to an automatic transmission for a motor vehicle, and more particularly to a hydraulic control system for an automatic transmission of multiple clutch type.
2. Description of the Prior Art
The official gazette of Japanese Patent Application Publication No. 31031/1988 discloses a hydraulic control system for an automatic transmission of multiple clutch type for use in a motor vehicle, the automatic transmission effecting so-called "clutch-to-clutch gear shifts". The automatic transmission has two power transmission shafts each of which transmits power from a driving source to driving wheels. The respective power transmission shafts are connected or disconnected on power transmission paths by correspondent clutches. The clutches are engaged or released by clutch oil pressures which are controlled by solenoid valves etc. Thus, one of the clutches is released from its engaged state, and the other is engaged from its released state, whereby the clutches are selectively brought into engagement so as to attain a plurality of gear shift stages.
In the hydraulic control system for the multiple clutch type automatic transmission as explained above, the solenoid valve for controlling the clutch oil pressure is usually set "normally-open". This is intended to prevent the drawback that, when the solenoid valve has failed, the automatic transmission falls into its neutral state, so the motor vehicle becomes incapable of drive.
The normally-open solenoid valve, however, poses the problem that, when it has failed due to breaking of wire or the like, the two clutches might come into multiple engagement to simultaneously have power transmission capabilities and to lock the motor vehicle.